Чамберс К., Холлидей А.К. Современная неорганическая химия, 1975

286 G R O U P VI

A12O3 + 6OH~ + 3H2O -> 2[AI(OH)6]3-

Notice that the acidic character is associated with the ability of aluminium to increase its covalency from three in the oxide to six in the hydroxoaluminate ion, [A1(OH)6]3~; the same ability to increase covalency is found in other metals whose oxides are amphoterie, for example

ZnO -> [Zn(OH)4]2~ or [Zn(OH)6]4~ PbO -» [Pb(OH)4]2- or [Pb(OH)6]4~

HIGHER OXIDES

Variable oxidation state is also exhibited in the oxides themselves among metals in this region of electronegativity. Thus lead, for example, forms the monoxide PbO (+ 2) and the dioxide PbO2 ( + 4) (thecompound Pb3O4 is not a simple oxide but is sometimes called a 'compound' oxide). Similarly, manganese gives the oxides MnO and MnO2.

Although the dioxides are oxidising agents, for example

PbO2 + 4HC1 -> PbCl2 + 2H2O + C12T

the oxidising power lies in the higher valency or oxidation state of the metal, not in the presence of more oxygen (distinction from peroxides, see below).

The more noble metals (for example copper, mercury and silver) can form oxides, and exhibit variable oxidation state in such compounds (for example Cu2O, CuO),but it is not easy to prepare such oxides by direct action of oxygen on the metal, and elevated temperatures are necessary. Moreover, in the case of silver and mercury, loss of oxygen from the oxide by heating is easy. The oxides are, however,basic (for example Ag2O -> Ag+ ,CuO -» Cu2 + in acids).

ACIDIC OXIDES

The other more electronegative elements are non-metals and form oxides which are entirely covalent and usually acidic. For example, sulphur yields the oxides SO2 and SO3, dissolving in bases to form the ions SOf ~ and SOj" respectively. A few non-metallic oxides are often described as neutral (for example carbon monoxide and dinitrogen oxide) because no directly related acid anion is known to exist.

GROUP V! 287

The two oxides formed with hydrogen, H2O and H2O2, have already been discussed, but it should be emphasised that hydrogen peroxide and the peroxides formed from it contain the —O—O— linkage. The oxidising power of these peroxides lies in the oxygen of the peroxo-group, unlike the dioxides (see above).

SULPHIDES

1. The alkali metal sulphides

These are ionic solids and can exist as the anhydrous salts (prepared by heating together sulphur with excess of the alkali metal) or as hydrates, for example Na2S.9H2O. Since hydrogen sulphide is a weak acid these salts are hydrolysed in water,

S2~ 4- H2O-»HS'

HS~ + H2 O-»H2 S-h OH~

and smell of hydrogen sulphide. Aqueous solutions of these salts are conveniently prepared by the action of hydrogen sulphide on the alkali metal hydroxide ; if excess hydrogen sulphide is used the hydrogensulphide is formed, for example NaHS. Solutions of these sulphides can dissolve sulphur to give coloured polysulphides, for example Na2S4 containing anionic sulphur chains.

2. The sulphides of alkaline earth metals

These are similar to those of the alkali metals but are rather less soluble in water. However, calcium sulphide, for example, is not precipitated by addition of sulphide ions to a solution of a calcium salt, since in acid solution the equilibrium position

H2S + Ca2+ ^CaS + 2H+

is very much to the left and in neutral, or alkaline solution the soluble hydrogensulphideis formed, for example

CaS + H?O -> Ca2+ + HS" + OH~

3. The sulphides of aluminium and chromium

These can be prepared by the direct combination of the elements.

288 GROUP V!

They are rapidly hydrolysed by water and the hydrolysis of solid aluminium sulphide can be used to prepare hydrogen sulphide:

A12S3 + 6H2O -> 2A1(OH)3 + 3H2St

Consequently they cannot be prepared by the addition of sulphide ions to a solution of the metal salt, the hydrated metal ions being so strongly acidic that the following reaction occurs, for example

2[A1(H2O)6]3+ + 3S2" -+ 2[Al(OH)3(H2O)3]i + 3H2St

The sulphides of most other metals

These are practically insoluble in water, are not hydrolysed and so may be prepared by addition of a sufficient concentration ofsulphide ion to exceed the solubility product of the particular sulphide. Some sulphides, for example those of lead(II), copper(II) and silver(I), have low solubility products and are precipitated by the small concentration of sulphide ions produced by passing hydrogen sulphide through an acid solution of the metal salts; others for example those of zinc(II), iron(II), nickel(II) and cobalt(II) are only precipitated when sulphide ions are available in reasonable concentrations, as they are when hydrogen sulphide is passed into an alkalinesolution.

Many of these sulphides occur naturally, for example iron(II) sulphide, FeS (magnetic pyrites), and antimony(III) sulphide, Sb2S3 (stibnite). They can usually be prepared by the direct combination of the elements, effected by heating, but this rarely produces a pure stoichiometric compound and the product often contains a slight excess of the metal, or of sulphur.

SELENIDES AND TELLURIDES

These closely resemble the corresponding sulphides. The alkali metal selenides and tellurides are colourless solids, and are powerful reducing agents in aqueous solution, being oxidised by air to the elements selenium and telluriumrespectively (cf.the reducingpower of the hydrides).

OXIDES AND OXO-ACIDS AND THEIR SALTS

The elements, sulphur, selenium and tellurium form both diand tri-oxides. The dioxides reflect the increasing metallic character of

GROUP VI 289

the elements. At room temperature, sulphur dioxide is a gas, boiling point 263 K, selenium dioxide is a volatile solid which sublimes at 588 K under 1 atmosphere pressure, and tellurium dioxide is a colourless, apparently ionic, crystalline dimorphic solid.

Sulphur

SULPHUR DIOXIDE, SO2

Sulphur dioxide is formed together with a little of the trioxidewhen sulphur burns in air:

S + O2 -> SO2

2S + 3O2 -* 2SO3

It can be prepared by the reduction of hot concentrated sulphuric acid by a metal. Copper is used since it does not also liberate hydrogen from the acid:

Cu + 2H2SO4 -> CuSO4 + 2H2O + SO2T

The equation is not strictly representative of the reaction for the acid is reduced further and a black deposit consisting of copper(I) and copper(II) sulphides is also produced.

Sulphur dioxide is also produced by the action of an acid (usually concentrated sulphuric since it is involatile) on a sulphite or hydrogensulphite, for example

2HSO3- + H2SO4 -> SOJ- + 2H2O 4- 2SO2t

On the industrial scale it is produced in large quantities for the manufacture of sulphuric acid and the production methods are dealt with later. It was once estimated that more than 4 000 000 tons of sulphur dioxide a year entered the atmosphere of Britain from the burning of coal and oil.

The molecule of sulphur dioxide has a bent structure. Both S—O distances are equal and short and since sulphur can expand its outer quantum level beyond eight, double bonds between the atom» are likely;i.e.

290 GROUP V!

Liquid sulphur dioxide as a solvent

Liquid sulphur dioxide is a solvent for a number of substances, for example iodine, sulphur, some sulphites, potassium iodide and sulphur dichloride oxide, SOC12 (see below). The liquid can be assumed to ionise slightly, thus:

2H2 O^ H3O+ + O H ~

Hence, for example, sulphur dichloride oxide behaves as an kacicT and a sulphite as a *base' thus :

SOC12 +	Na2SO3 ~+2NaCli +		2SO'2
S02+ 4- 2Cr 2Na+ + SOi" -»		salt	solvent
acid	base	(insoluble)

Properties of sulphur dioxide

Sulphur dioxide is oxidised by chlorine in the presence of charcoal or camphor to give sulphur dichloride dioxide (sulphuryl chloride),

SO2C12;

SO2 + C12 -> SO2C12

Dioxides and peroxides oxidise it to yield sulphates:

PbO2 + SO2 -> PbSO4

Na2O2 + SO2 -> Na2SO4

Sulphur dioxide is an acidic oxide and dissolves readily in water, and in alkalis with which it forms salts:

NaOH + SO2 -> NaHSO3

sodium hydrogensuiphite

2NaOH + SO2 -» Na2SO3 + H2O

sodium sulphite

Although sulphur dioxide, as a gas, is a reducing agent in the sense that it unites with oxygen, free or combined (for example in dioxides or peroxides) most of its reducing reactions in aqueous solution are better regarded as reactions of 'sulphurous acid' (in acid solution), or the sulphite ion (in alkaline solution).

G R O UP VI 291

'SULPHUROUS ACID'

The solution obtained when sulphur dioxide dissolves in water has long been thought to contain unionised sulphurous acid, H2SO3, but more probably contains hydrated sulphur dioxide (cf. NH3 solution, p. 217). The solution behaves as a dibasic acid, i.e.

SO2(aq) -f 2H2O ^ H3O+ HSO3~:

K = 1.6 x 10"2 moll-1 at 298 K

HSO3~ 4- H2O ^ H3O* 4- SOi' :

Ka = 6.2 x 1(T8 mol 1~ i at 298 K.

The sulphite ion, SOf ~, has a pyramidal structure and the short S—O bond length suggests the presence of double bonding, i.e.

t \

Two important redox potentials for reduction by sulphur dioxide in aqueous solution are :

Acid:SOj-(aq)	H3O+ -¥ 2e~ -»
	4H2SO3'(aq) -h 5H2O : £^ = 4- 0.17 V
Alkali: SO|~(aq) -f	H2O -f 2e~ ->
	SO|~ (aq) -h 2OH "(aq) : E^ = - 0.93V.

Some important reducing reactions are given below ;forsimplicity, the reducing entity is taken to be SOl~ in all cases.

1.Sulphites react with molecular oxygen (or air) to give sulphates, a reaction catalysed by certain ions (for example Fe2+ , Cu2 +, arsenate(III) ion, AsOl") and inhibited by, for example, phenol, glycerol and tin(II) ions, Sn2 + :

2.Sulphites react with oxidising agents, for example manganate(VII) and dichromate(VI) :

4-	+ 3H2O
Cr2Of~	-h 4HO

orange

green

292 G R O U P VI

This reaction is a useful test for a sulphite or for moist sulphur dioxide, which turns 'dichromate paper' (filter paper soaked in potassium dichromate) from yellow to green.

3. Sulphites are oxidised by chlorine water and solutions containing chloric(I) (hypochlorous)acid or the chlorate(I) (hypochlorite) ion

C12 + SO§~ + H2O ~» 2CP 4- SOr + 2H+

or

ocr + sor -»cr + soj-

4. Iron(III) is reduced to iron(II) by sulphites:

2Fe3+ -h SOI" 4- H2O -> 2Fe2+ + 2H -f SO^

In the presence of strong reducing agents the sulphite ion acts as an oxidising agent; some examples are:

1. The oxidation of hydrogen sulphide to sulphur:

2H2S 4- SO^ + 2H+ -> 3H2O + 3S|

2. In strongly acid solution, substances which are normally reducing agents reduce sulphur dioxide solution or sulphites, for example iron(II) and zinc:

4Fe2+ 4- SOr	+ 6H+ -»4Fe3+ 4- S| + 3H2O
2SOl~ + Zn -f 4H+ -* S2Oj" -f Zn2+ + 2H2O
(dust)	(dithionite)

If a solid sulphite is heated with zinc dust (or carbon) the sulphite is reduced to sulphide:

Na.SO, + 3Zn -* Na.S + 3ZnO

Uses

The reducing action of sulphurous acid and sulphites in solution leads to their use as mild bleaching agents (for example magenta and some natural dyes, such as indigo, and the yellow dye in wool and straw are bleached). They are also used as a preservative for fruit and other foodstuffs for this reason. Other uses are to remove chlorine from fabrics after bleaching and in photography.

GROUP VI 293

SULPHITES AND HYDROGENSULPHITES

When a saturated solution of sulphur dioxide is titrated against approximately 2 M sodium hydroxide solution the following pH curve is obtained (Figure 10.4):

,4

12

10

Q.

8	10	12	14	16	18	20
		cm3	2M No OH

Figure 10.4. Titration of 25 cm3 of saturated aqueous sulphur dioxide with 2 M sodium hydroxide at 298 K

The reaction completed on addition of 7.9 cm3 is

SO2(aq) + NaOH -» NaHSO3

sodium hydrogensulphite

and after 15.8cm3

SO2(aq) 4- 2NaOH ^ Na2SO3 4- H2O

sodium sulphite

Evaporation and crystallisation of the sodium sulphite solution gives crystals of the heptahydrate Na2SO3.7H2O. However, on evaporation of the hydrogensulphite solution, the solid obtained is chiefly sodium pentaoxodisulphate(IV) (smetabisulphite') Na2S2O5, and contains little ifany of the hydrogensulphite. However, the hydrogen sulphite ion is obtained when the solid redissolves in water:

Na2S?O H2O

HSO

294 GROUP VI

Alternatively these salts can be prepared by first saturating a known volume of alkali with sulphur dioxide, giving a solution of the hydrogensulphite, from which sulphite can be prepared by the addition of a second equal volume of alkali.

Properties

The redox properties have already been considered. A number of reactions of soluble (alkali metal) sulphites are noteworthy:

1. On boiling a solution of a sulphite with sulphur a thiosulphate(VI)* is formed, and sulphur 'dissolves':

SOi~ + S -> S2Or (e.g. Na2S2O3)

thiosulphateCVI)

Sodium thiosulphate is an important reducing agent used in volumetric analysis for the estimation of iodine:

I2 + 2S2Or ~+2F + S4Oi-

tetrathionate ion

It is used as the Tixer' in photography under the name 'hypo'.

2. Addition of barium chloride precipitates white barium sulphite:

Ba2+ + SOr-»BaSO3l

Barium sulphite is soluble in dilute hydrochloric acid unlike barium sulphate which is insoluble. Hence this reaction, and the evolution of sulphur dioxide on addition of an acid, distinguishes a sulphite from a sulphate.

3. Sodium hydrogensulphite, when freshly prepared, reacts with aldehydes to form crystalline addition compounds, for example

H	/H
HSO7 + CH.C	CH,C—OH
O	SO3
ethanal
(acetaldehyde)

This reaction is used in organic chemistry to separate an aldehyde from, for example, an ester.

* The thiosulphate ion has the structure [S=SO3]2~; the oxidation state of the central sulphur atom is + 6.

G R O U P VI 295

SULPHUR TRIOXIDE

Sulphur trioxide was first prepared by heating iron(III) sulphate :

Fe2(SO4)3 -» Fe2O3 + 3SO3

It is also obtained by the dehydration of concentrated sulphuric acid with phosphorus(V) oxide:

2H2SO4 + P4O10 -> 4HPO3 + 2SO3 and the thermal decomposition of iron(II) sulphate :

2FeSO4 -* Fe2O3 + SO2 -h SO3

iron(II) iron(II) sulphate oxide

In the laboratory it is commonly prepared by the reaction between sulphur dioxide and oxygen at high temperature in the presence of a platinum catalyst :

2SO2 + O2 ^ 2SO3

(This is the basis of the industrial manufacture of sulphuric acid and is dealt with on p. 296.)

Sulphur trioxide can be collected as a white solid in a receiver surrounded by a freezing mixture of ice and salt.

Properties

In the vapour state, sulphur trioxide has the formula SO3. The molecule is planar with all the S—O bonds short and of equallength. The structure can be represented simply as

O

A

but is probably a resonance hybrid of several forms. Solid sulphur trioxide exists in at least two modifications, the a and j3 forms. The a form is an ice-like transparent solid consisting of rings of formula S3O9 (shown geometrically below):

a - SO3

O O